Gas turbine engines create combustion gas which is expanded through a turbine to generate power. The combustion gas is often heated to a temperature which exceeds the capability of the substrates used to form many of the components in the turbine. To address this, the substrates are often coated with thermal barrier coatings (TBC) and also often include cooling passages throughout the component. A cooling fluid such as compressed air created by the gas turbine engine's compressor is typically directed into an internal passage of the substrate. From there, it flows into the cooling passages and exits through an opening in the surface of the component and into the flow of combustion gas.
Certain turbine components are particularly challenging to cool, such as those components having thin sections. The thin sections have relatively large surface area that is exposed to the combustion gas, but a small volume with which to form cooling channels to remove the heat imparted by the combustion gas. Examples of components with a thin section are those having an airfoil, such as turbine blades and stationary vanes. The airfoil usually has a thin trailing edge.
Various cooling schemes have been attempted to strike a balance between the competing factors. For example, some blades use structures in the trailing edge, where cooling air flowing between the structures in a first row is accelerated and impinges on structures in a second row. A faster flow of cooling fluid will more efficiently cool than will a slower flow of the same cooling fluid. This may be repeated to achieve double impingement cooling, and repeated again to achieve triple impingement cooling, after which the cooling air may exit the substrate through an opening in the trailing edge, where the cooling air enters the flow of combustion gas passing thereby. The impingement not only cools the interior surface of the component, but it also helps regulate the flow. In particular it may create an increased resistance to flow along the cooling channel and this may prevent use of excess cooling air.
For cost efficient cooling design the trailing edge is typically cast integrally with the entire blade using a ceramic core. The features and size of the ceramic core are important factors in the trailing edge design. A larger size of a core feature makes casting easier, but the larger features are not optimal for metering the flow through the crossover holes to achieve efficient cooling. In the trailing edge, for example, since cavities in the substrate correspond to core material, a crossover holes between the adjacent pin fins in a row corresponds to sparse casting core material in that location of the casting. This, in turn, leads to fragile castings that may not survive normal handling. To achieve acceptable core strength the crossover holes must exceed a size optimal for cooling efficiency purposes. However, the crossover holes result in more cooling flow which is not desirable for turbine efficiency. Consequently, there remains room in the art for improvement.